I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method for executing multi-mode handoffs in a cellular system through preliminary time alignment of a mobile station.
II. Description of the Related Art
In the field of cellular communications, systems employing both analog and digital systems have been developed. Systems operating in the analog mode are typically of a frequency modulation type, an example of which is the system known as the Advanced Mobile Phone Service (AMPS), as prescribed in Electronic Industry Association standard EIA/TIA-553 "Mobile Station-Land Station Compatibility Specification". Systems operating in the digital mode may be of different types of digital modulation such as time division multiple access system (TDMA) or a spread spectrum multiple access type, employing a frequency hopping (FH) or direct sequence spread spectrum modulation mode. Digital multiple access systems employing direct sequence spreading are commonly referred to as Code Division Multiple Access (CDMA) systems. An example of a TDMA system is one that is described in the Telecommunications Industry Association/Electronic Industry Association Interim Standard TIA/EIA/IS-54 "Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard", and the revisions thereto (TIA/EIA/IS-54A and TIA/EIA/IS-54B). Another example of a TDMA system is the system known as the Global System for Mobile Communication (GSM). An example of a CDMA system is one that is set forth in the Telecommunications Industry Association/Electronic Industry Association Interim Standard TIA/EIA/IS-95 "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", and the revisions thereto (TIA/EIA/IS-95A). Although these standards are typically defined for frequency spectrum specific to cellular communications, the analog and digital techniques are also applicable to other frequencies such as those defined for personal communication services (PCS).
Dual mode cellular communication mobile stations or subscriber units are capable of operating in both a standard analog mode and a digital mode. Within such a system, a need may arise to handoff from analog mode to digital mode during an active call or vice versa. Analog to digital system handoffs are not restricted to those within the cellular frequency band, but may also occur between an analog cellular system and a digital PCS system.
Regardless of the systems involved, a prolonged discontinuity in service during the handoff from analog mode to digital mode is unacceptable to the users of the system. A quality system must therefore incorporate a dual mode handoff method that causes a minimum interruption of service. However, in standard analog mode time alignment of a mobile station to a predetermined system time is not required, and no analogous function is currently available. In digital mode, time alignment is critical and required for communication between the mobile station and the base station. This dissimilarity between the two modes can cause delays in handoff from analog mode to digital mode.
Within the current analog cellular phone system, a handoff of a mobile station call can be performed between base stations during an active call. One handoff criteria which initiates a handoff is the strength of the mobile station signal arriving at the base station. Initiation of a handoff by the base station begins when the base station with which a mobile station is communicating receives from the mobile station a message indicative of insufficient mobile station received signal strength. The base station sends a request to a system controller or mobile switching center (MSC) for a handoff of the call. The system controller polls the neighboring base stations to ascertain an estimate of the signal strength of the mobile station at neighboring base stations, and, from this information, determines a candidate base station. The original base station informs the mobile station of the frequency and various other parameters of the candidate base station, and directs the mobile station to handoff. The call is then transferred from the original base station to the candidate base station, which becomes the new active base station. It should be noted that the procedure above might vary depending on the manufacturer of the equipment performing the functions.
The handoff of a mobile station call in a digital system, such as a CDMA system or Time Division Multiple Access (TDMA), is initiated either by a base station or by a mobile station. In a typical CDMA digital system, neighboring base stations use the same frequency for transmission, thus allowing a mobile station to monitor signal strengths of neighboring base stations during an active call. A mobile station sends a message when one of the neighboring base stations' signal arrives at the mobile station at a level exceeding some predetermined threshold, thereby initiating a handoff procedure. The system controller commands the neighboring base station to establish communication with the mobile station if resources are available at the neighboring base station. The system controller relays a message to the mobile station, through the active base station, directing the mobile station to establish communication with the neighboring base station. The mobile station establishes communication with the neighboring base station without breaking the call with the active base station. The mobile station can maintain the call with two base stations until one of the base stations' signal strength falls below a predetermined level. Further information on handoffs within a digital CDMA system is disclosed in a U.S. Pat No. 5,101,501 entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", the disclosure of which is incorporated by reference herein, which is assigned to the Assignee of the present invention. Also, further information on handoffs within a digital CDMA system is disclosed in U.S. Pat. No. 5,267,261, entitled "MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM", the disclosure of which is also incorporated by reference herein, and also assigned to the Assignee of the present invention.
A handoff from digital mode to analog mode is typically easier than the handoff from analog mode to digital mode. A digital mode to analog mode handoff could be initiated by either a mobile station or a base station on the basis of the mobile station having exited the coverage area of the digital system. In digital mode the base station can transmit the necessary information about the analog system including, for example, the new frequency assignment, supervisory audio tones (SAT), and voice mobile attenuation code (VMAC) information as dictated by the system controller. Similar to handoff within an analog system, the call can then be transferred from the base station operative in digital mode to an analog mode base station.
As a digital system is initially deployed covering only certain localities within an operating system, a need may arise to perform a handoff from analog mode to digital mode. For example, a digital system may be deployed in a metropolitan city but not deployed in the surrounding suburban areas. A user may initiate an analog call as he leaves the suburban area driving towards the city. As the metropolitan area is approached, the heavily laden analog system may not have the capacity to accept the transfer of the call, hence necessitating that the call be terminated prior to completion (i.e., "dropped"). If the dual mode system were capable of transferring the call from the analog system to the higher capacity digital system, it would be unnecessary to drop the call.
The need for handoff from analog mode to digital mode may also arise within the context of private branch exchange (PBX). A digital PBX system integrated into an office building might be surrounded by an analog system outside of the building. A handoff from the outside analog system to the inside digital system would take place as the user entered the building while maintaining an active call. If the system could not perform such a handoff, the call could be dropped due to the inability of the analog signals to penetrate the walls of the building and maintain signal levels capable of supporting continued communication.
A complication to the handoff from analog mode to digital mode is that digital mode requires a finite time duration for the mobile station to synchronize (acquire time alignment). The acquisition time typically varies as a function of: the system architecture of the digital system to be deployed, the signal to noise level at the mobile station at the time of attempted acquisition, the accuracy of the initial guess, and as a function of the acquisition algorithm used. In a typical CDMA system, the average system acquisition time is several seconds. The acquisition time in TDMA systems is generally less than in CDMA systems. However, in either instance the discontinuity of service resulting from the requisite acquisition time may be unacceptable in a voice system.